This application claims priority of Japanese patent application number 2000-048671, filed Feb. 25, 2000.
This invention relates to an optical transmission line, and more particularly relates to an optical transmission line for realizing ultra long haul repeaterless transmission or an ultra long haul repeater span.
In ultra long haul repeater span transmission, signal-to-noise ratio (SNR) can be maintained higher as input power for each fiber span increases. However, when the optical input power increases, transmission characteristics deteriorate because the input power is limited by stimulated Brillouin scattering (SBS) and a nonlinear optical effect increases.
In the meanwhile, the following two configurations have been proposed, one is to extend a spectrum through low-frequency micro-signal modulation in order to suppress the SBS and the other is to use a fiber with a mode field diameter (MFD) as large as approximately 10 xcexcm (e.g. a single mode fiber (SMF) having a zero dispersion wavelength at a 1.3 band) in order to reduce the nonlinear optical effect even if the optical input power is high. For instance, U.S. Pat. No. 6,157,754 and Japanese Laid-Open Patent Publication No. 10(1998)-322283 (U.S. patent application Ser. No. 09/064,020) disclose a configuration in which an optical fiber with a large effective core area and a large dispersion slope is disposed on an input side of signal light, and an optical fiber with a small effective core area and a small dispersion slope is disposed on an output side of the signal light.
Also, such configuration is well known that uses Raman amplification as a method to lengthen a repeaterless optical transmission distance or a repeater span.
However, when a single mode fiber (SMF) is used at a 1.5 xcexcm band, accumulated chromatic dispersion becomes large as its chromatic dispersion is large (approximately xe2x88x9218 ps/nm/km). Accordingly, the configuration requires a large number of dispersion compensating fibers (DCF). In addition, it is difficult to obtain large Raman gain since the MFD of the single mode fiber is large as approximately 10 xcexcm.
Generally, in order to obtain large Raman gain through a fiber Raman amplifier of backward, a fiber with a small MFD should be disposed on an output side of signal light wherein the pumping light power is large. However, in a case that the MFD is too small, fiber loss at a pumping wavelength band increases, and so large Raman gain cannot be realized and transmission characteristics do not improve.
It is therefore an object of the present invention to provide an optical transmission line to realize more satisfactory transmission characteristics by using Raman amplification.
An optical transmission line according to the invention is composed of a first optical fiber to transmit signal light and have a first effective core area and a first chromatic dispersion value, a second optical fiber to transmit the signal light from the first optical fiber and have a second effective core area smaller than the first effective core area and a second chromatic dispersion value which is either smaller than or negative-polarity of the first chromatic dispersion value, a pumping light source to generate pumping light to cause Raman amplification in the second optical fiber, and an optical coupler to apply the pumping light from the pumping light source into the second optical fiber from the back.
In the above configuration, by reducing loss of the signal light through the Raman amplification, the transmission of longer distance is realized. Also, since the effective core area of the optical fiber disposed on the input side of the signal light is larger than that of the optical fiber disposed on the output side of the signal light, the nonlinear effect is reduced and thus it becomes possible to input the signal light having even greater power.
When effective Raman gain and background noise are considered, the effective core area of the second optical fiber should preferably be from 55 xcexcm2 to 70 xcexcm2.
When input power of the first optical fiber is expressed as Pin (dBm), a total loss of the first and second optical fibers is expressed as xcex1 (dB), Raman pumping power is expressed as Pp (dBm), and a total length of the first and second optical fibers is expressed as L (km), it is provided that y=(Pinxe2x88x92xcex1)/(Ppxc2x710 Log L). When the ratio of the second optical fiber to L is expressed as x (0xe2x89xa6xxe2x89xa61), y should be preferably no more than 6.63xc3x9710xe2x88x922x+0.123 and no less than 6.63xc3x9710xe2x88x922xxe2x88x920.277. With this, a satisfactory S/N ratio is obtained.
When y is practically equal to 6.63xc3x9710xe2x88x922xxe2x88x920.077, the maximum S/N ratio is obtained.